In a previous experimental investigation of the effects of residual gas nonuniformity on S.I. engine combustion variability, it was found that eliminating residual gas nonuniformity by skip firing had no detectable impact on the flame development process, but nonetheless caused IMEP fluctuations to drop by about half under very light load conditions. Under further investigation, it has been determined that the observed IMEP fluctuations, particularly for optimally-phased cycles, are controlled by cyclic variations in the amount of fuel burned per cycle. Real-time sampling of the hydrocarbon concentration in the exhaust port has shown that the variation in fuel burned per cycle is not primarily due to variations in combustion completeness, and must therefore be attributed to variations in the amount of fuel trapped within the cylinder prior to combustion. Several mechanisms for this variation were identified, all of which are plausible but none of which are likely to dominate: variations in fuel quantity left in the cylinder from the previous cycle; variations in the fluid dynamics of the intake process; fresh charge displacement due to variations in residual gas temperature; variations in leakage through valves; and fluctuations in crevice effects and blow-by.